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K Eggan

Publications and source records attributed to K Eggan.

7 recordsLinked to original sources
PubMed2001

Nuclear cloning and epigenetic reprogramming of the genome.

Cloning of mammals by nuclear transfer (NT) results in gestational or neonatal failure with at most a few percent of manipulated embryos resulting in live births. Many of those that survive to term succumb to a variety of abnormalities that are likely due to inappropriate epigenetic reprogramming. Cloned embryos derived from donors, such as embryonic stem cells, that may require little or no reprogramming of early developmental genes develop substantially better beyond implantation than NT clones derived from somatic cells. Although recent experiments have demonstrated normal reprogramming of telomere length and X chromosome inactivation, epigenetic information established during gametogenesis, such as gametic imprints, cannot be restored after nuclear transfer. Survival of cloned animals to birth and beyond, despite substantial transcriptional dysregulation, is consistent with mammalian development being rather tolerant to epigenetic abnormalities, with lethality resulting only beyond a threshold of faulty gene reprogramming encompassing multiple loci.

Animals↗
PubMed2001

Epigenetic instability in ES cells and cloned mice.

Cloning by nuclear transfer (NT) is an inefficient process in which most clones die before birth and survivors often display growth abnormalities. In an effort to correlate gene expression with survival and fetal overgrowth, we have examined imprinted gene expression in both mice cloned by nuclear transfer and in the embryonic stem (ES) cell donor populations from which they were derived. The epigenetic state of the ES cell genome was found to be extremely unstable. Similarly, variation in imprinted gene expression was observed in most cloned mice, even in those derived from ES cells of the same subclone. Many of the animals survived to adulthood despite widespread gene dysregulation, indicating that mammalian development may be rather tolerant to epigenetic aberrations of the genome. These data imply that even apparently normal cloned animals may have subtle abnormalities in gene expression.

Animals↗
PubMed2001

Hybrid vigor, fetal overgrowth, and viability of mice derived by nuclear cloning and tetraploid embryo complementation.

To assess whether heterozygosity of the donor cell genome was a general parameter crucial for long-term survival of cloned animals, we tested the ability of embryonic stem (ES) cells with either an inbred or F(1) genetic background to generate cloned mice by nuclear transfer. Most clones derived from five F(1) ES cell lines survived to adulthood. In contrast, clones from three inbred ES cell lines invariably died shortly after birth due to respiratory failure. Comparison of mice derived from nuclear cloning, in which a complete blastocyst is derived from a single ES cell, and tetraploid blastocyst complementation, in which only the inner cell mass is formed from a few injected ES cells, allows us to determine which phenotypes depend on the technique or on the characteristics of the ES cell line. Neonatal lethality also has been reported in mice entirely derived from inbred ES cells that had been injected into tetraploid blastocysts (ES cell-tetraploids). Like inbred clones, ES cell-tetraploid pups derived from inbred ES cell lines died shortly after delivery with signs of respiratory distress. In contrast, most ES cell-tetraploid neonates, derived from six F(1) ES cell lines, developed into fertile adults. Cloned pups obtained from both inbred and F(1) ES cell nuclei frequently displayed increased placental and birth weights whereas ES cell-tetraploid pups were of normal weight. The potency of F(1) ES cells to generate live, fertile adults was not lost after either long-term in vitro culture or serial gene targeting events. We conclude that genetic heterozygosity is a crucial parameter for postnatal survival of mice that are entirely derived from ES cells by either nuclear cloning or tetraploid embryo complementation. In addition, our results demonstrate that tetraploid embryo complementation using F(1) ES cells represents a simple, efficient procedure for deriving animals with complex genetic alterations without the need for a chimeric intermediate.

Animals↗
PubMed2000

X-Chromosome inactivation in cloned mouse embryos.

To study whether cloning resets the epigenetic differences between the two X chromosomes of a somatic female nucleus, we monitored X inactivation in cloned mouse embryos. Both X chromosomes were active during cleavage of cloned embryos, followed by random X inactivation in the embryo proper. In the trophectoderm (TE), X inactivation was nonrandom with the inactivated X of the somatic donor being chosen for inactivation. When female embryonic stem cells with two active X chromosomes were used as donors, random X inactivation was seen in the TE and embryo. These results demonstrate that epigenetic marks can be removed and reestablished on either X chromosome during cloning. Our results also suggest that the epigenetic marks imposed on the X chromosomes during gametogenesis, responsible for normal imprinted X inactivation in the TE, are functionally equivalent to the marks imposed on the chromosomes during somatic X inactivation.

Alleles↗